The present invention relates to blind fasteners employed to join composite materials.
A particular type of such a fastener is a blind rivet, well known in the prior art, used to fasten components when only one side of the workpiece is accessible. These fasteners generally comprise a stem or mandrel having a buckle-forming head at one end and serrations at the other end for engagement by a pulling tool; a tubular body surrounding the mandrel and having a flanged head; and, a locking collar encircling the mandrel near the body head. In use, the head portion of the mandrel and the surrounding body together are inserted through a hole in the workpiece. A riveting tool including a pulling head is used to translate the mandrel axially away from the workpiece. During such translation, the buckle-forming elements of the mandrel head expand the rivet body laterally to form a buckle on the blind side of the workpiece. The pulling head then forces the locking collar into a groove in the mandrel to lock the headed, accessible end of the body to the stem. Finally, the stem portion of the mandrel extending from the workpiece is broken off to complete the installation.
Another type of blind fastener has a threaded body to receive a setting screw or core bolt, with a separate buckle sleeve as the expandable element. The sleeve is expanded on the blind side of the workpiece by tightening the threaded core bolt and fastener body. The fastener body has a manufactured head for bearing on the setting side of the workpiece. This head may have wrenching flats for use of a wrenching tool to prevent fastener body rotation during setting. The manufactured head may protrude or be flush. The fastener body has a shank with a diameter for substantially complete occupancy of the aligned holes in the workpieces. A blind side end of the fastener body, the end opposite the manufactured head, is a nose which externally tapers to provide an expansion surface over which the sleeve expands and forms against the workpiece.
The sleeve is cylindrical and has an external diameter no greater than the diameter of the fastener body so that the sleeve passes through the aligned holes in the workpieces, with an internal diameter for receipt of the core bolt. The core bolt head has a diameter no greater than the diameter of the sleeve for passage to the blind side and a radial shoulder for bearing on the end of the sleeve. The core bolt has wrenching flats or other means on the setting side for tightening in the female threads of the fastener body. As the core bolt is rotated in the thread and moved axially relative to the fastener body, the bolt head bears on the sleeve and the sleeve is forced over the nose of the fastener body and expanded against the blind side of a joint. Load determining means such as a breakneck in the bolt can break to stop tightening and determine the amount of clamp-up force.
In joining materials such as aluminum, damage to the hidden, or "blind" side of the workpiece by these conventional fastener assemblies had to be considered, but was not of major concern due to the inherent strength and reduced frangibility of aluminum and other materials. Such damage generally occurred during the contact of the buckle sleeve with the blind side of the workpiece prior to buckle formation. Without the formed buckle to dissipate the force of the pulling mandrel or bolt along a large area of the workpiece, the relatively smaller area buckle sleeve contacted the workpiece during buckle formation, thereby transmitting the pulling force of the mandrel or bolt onto a relatively small area of the workpiece.
However, the use and development of composite materials such as graphite or fiber/epoxy matrix has increased. Therefore, the problem of damaging the more sensitive composites from the blind side during buckle formation is of concern, as these composites are more prone to damage when such concentrated force is transmitted. While conventional fastener assembly configurations may prove useful in connection with certain materials and in certain workpiece stress situations, there exists a need for a fastener assembly which provides substantially reduced stress on a workpiece, specifically workpieces made of composite material, during fastener setting and buckle formation. Reduction of workpiece stress is especially important in aerospace applications which presently use a variety of newly developed composite materials.
A number of blind fastener assemblies for composite materials have been developed, with the primary focus on formation of the buckle in the buckle sleeve substantially prior to contact with the workpiece. Forming the buckle prior to workpiece contact dissipates the force of the pulling mandrel along a larger area of the blind side of the workpiece, thereby reducing the chance that the composite workpiece will fail during compression. One such fastener is U.S. Pat. No. 4,312,613 to Binns, disclosing a multiple shoulder type rivet assembly.
In the multiple shoulder configuration, the sleeve and rivet body are specifically configured such that, as the mandrel is pulled through the workpiece, the rivet sleeve begins to slide and expand laterally over the dual-domed tail portion of the body member, substantially forming two buckles at the shoulder areas prior to sleeve contact with the workpiece. However, a problem with this assembly is that the action on which the dual buckle formation and propagation depend requires multiple intermediate configurations of the buckle sleeve during buckle formation, thereby increasing the likelihood of incomplete or discontinuous buckle formation prior to workpiece contact, thus increasing the likelihood of workpiece damage.
Furthermore, to the above shortcomings, the dual shoulder rivet requires a complex tail geometry on the rivet body and buckle sleeve, requiring expensive secondary manufacturing operations on the dual-domed tail end of the body member and the dual-shoulder buckle sleeve.